1) Field of the Invention
The present invention relates to a technique for digitally controlling operations of at least two system DC motors by pulse-width modulation (PWM). More particularly, the invention relates to a technique suitable for driving and controlling the motors of a conveyance robot (access mechanism) that conveys cartridges within a library apparatus and to a conveyance robot and library apparatus employing the technique.
2) Description of the Related Art
A typical library apparatus serves as a large-capacity external storage and has shelves in which numerous cartridges with magnetic tape as a storage medium are achieved. The storage medium in each cartridge can be automatically accessed to read or write data.
The library apparatus, in addition to the shelves for storing cartridges, is equipped with a plurality of decks for accessing the storage medium (magnetic tape) of each cartridge to read or write data, and a conveyance robot (access mechanism) for conveying cartridges between the shelves and decks.
In the above-described library apparatus, if a request to access a cartridge is accepted from a host, the access mechanism moves to a shelf housing the cartridge, then grasps that cartridge with the hand mechanism of the access mechanism and conveys it to a deck, and inserts the cartridge into the deck. In the deck, data is read from or written to the storage medium (magnetic tape). After the data processing, the cartridge removed from the deck is grasped again by the hand mechanism of the access mechanism, and with this access mechanism, it is conveyed to a housing shelf and housed at a predetermined position.
The access mechanism in a typical library apparatus is shown in FIG. 5 by way of example. The access mechanism 100 shown in the figure is made up of a hand mechanism 110, a horizontal moving mechanism 120, and a vertical moving mechanism 130, in order to convey a cartridge 200 within the library apparatus in the above-described manner.
The hand mechanism 110 consists of a gripping mechanism 111 for grasping a cartridge 200 and inserting and removing the cartridge 200 with respect to a deck or housing shelf, and a swivel mechanism 112 for swiveling the gripping mechanism 111.
The horizontal moving mechanism 120 is used for moving the hand mechanism 110 in a horizontal direction (X-axis direction), while the vertical moving mechanism 130 is used for moving both the hand mechanism 110 and horizontal moving mechanism 120 in a vertical direction (Y-axis direction). The moving mechanisms 120 and 130 are equipped with two system motors 51 and 52 (see FIG. 6), which are driven by pulse-width modulation (PWM).
The above-described access mechanism 100 is constructed so that the requested cartridge 200 grasped by the gripping mechanism 111 of the hand mechanism 110 is conveyed from a certain coordinate point (X0, Y0) to a target coordinate point (XP, YP) by the moving mechanisms 120 and 130 (e.g., it is conveyed from a housing shelf to a deck, or from a deck to a housing shelf).
Normally, the moving mechanisms 120 and 130 are operated simultaneously at the highest speed in order to move the cartridge 200 as fast as possible. That is, the two system motors 51 and 52, for operating the moving mechanisms 120 and 130, are digitally controlled by PWM so that they are driven simultaneously at peak power.
Referring to FIG. 6, there is shown a typical circuit that causes two system motors 51 and 52 to operate by PWM. As shown in the figure, the two system DC motors 51 and 52 are connected in parallel with the power supply 71 of a power supply module 70. These DC motors 51 and 52 are also connected in series with switches 61 and 62 so that they are driven by PWM. The power supply module 70, in addition to the power supply 71, is equipped with a smoothing circuit (LPF: Low-Pass Filter) 72.
The PWM signals S1 and S2 from a PWM-signal generation circuit 80 to be described later are supplied to the switches 61 and 62, which are turned on or off according to the states (high or low) of the PWM signals S1 and S2. For example, if the PWM signals S1 and S2 are high (H), the switches 61 and 62 are turned on so that power is supplied to the DC motors 51 and 52. Conversely, if the PWM signals S1 and S2 are low (L), the switches 61 and 62 are turned off so that power to the DC motors 51 and 52 is stopped.
Referring to FIG. 7, there is shown a conventional PWM-signal generation circuit 80 for supplying PWM signals S1 and S2 to switches 61 and 62. As shown in the figure, the PWM-signal generation circuit 80 is made up of a first PWM-signal generator (comparator; CMP) 81, a second PWM-signal generator (comparator; CMP) 82, and a counter 83.
In the PWM-signal generation circuit 80, a cycle set value T0 for determining the cycles T (see FIG. 8) of the PWM signals S1 and S2 is set and it is input to the counter 83. Also, a first duty set value d1 for determining a duty ratio for the PWM signal S1, and a second duty set value d2 for determining a duty ratio for the PWM signal S2, are set at cycles of T. The first and second duty set values d1 and d2 are input to the first and second comparators 81 and 82, respectively. The duty ratio for the PWM signal S1 or S2 is a ratio of the high state to the cycle T. As shown in FIG. 8, the duty ratio of the first PWM signal S1 is t1/T, t1′/T, and t1″/T, and the duty ratio of the second PWM signal S2 is t2/T, t2′/T, and t2″/T.
The counter 83 counts the number of clocks and outputs the count to the comparators 81 and 82 and is reset if it counts to the cycle set value T0. For instance, if the cycle set value T0 is 100, the counter 83 is reset if it counts clocks from 1 to 100. The counts that are output from the counter 83 to the comparators 81 and 82 are 1, 2, 3, . . . , and 100, which are repeated.
The comparators 81 and 82 compare the count from the counter 83 with the duty set values d1 and d2, and switch, according to the result of comparison, the PWM signals S1 and S2 from a high state to a low state, or from a low state to a high state. For example, the comparators 81 and 82 switch the PWM signals S1 and S2 from a high state to a low state when the count from the counter 83 exceeds the duty set values d1 and d2. More specifically, if the duty set values d1 and d2 are 50 (i.e., a duty ratio of 50%), the PWM signals S1 and S2 are in a high state when the count from the counter 83 is between 1 and 50 and are in a low state when the count is between 51 and 100. Also, if the duty set values d1 and d2 are 0 (a duty ratio of 100%), the PWM signals S1 and S2 are always in a high state. If the duty set values d1 and d2 are 100 (a duty ratio of 0%), the PWM signals S1 and S2 are always in a low state.
In this way, in the PWM-signal generation circuit 80, by suitably setting the cycle set value T0 and duty set values d1 and d2, the PWM signals S1 and S2 are generated at desired cycles T so the above-described two system motors 51 and 52 can be driven simultaneously at peak power.
The waveforms and temporal overlap of the PWM signals S1 and S2 generated by the PWM-signal generation circuit 80 shown in FIG. 7 are shown in FIG. 8. As shown in the figure, the PWM signals S1 and S2 generated by the PWM-signal generation circuit 80 shown in FIG. 7 rise simultaneously at predetermined cycles T and hold a high state at the duty ratios determined by the duty set values d1 and d2.
The periods that the PWM signals S1 and S2 rise simultaneously (i.e., the periods that the switches 61 and 62 are simultaneously turned on so that power is simultaneously supplied to the motors 51 and 52) are shown on the bottom row of FIG. 8. During the period the PWM signals S1 and S2 of two systems are simultaneously in a high state, load current of two systems will flow. However, since the temporal overlap of the PWM signals S1 and S2 is very short, it is common practice to suppress an adverse influence due to an instantaneous fluctuation in load by employing a bypass capacitor (not shown) in the power supply module 70. In such a case, the bypass capacitor can employ aluminum electrolytic capacitors that are low-cost and have large capacity.
Note that the technique of adjusting electric current when controlling motors used in a plurality of systems is disclosed, for example, in Japanese Laid-Open Patent Publication Nos. HEI 6-094342, HEI 8-105270, HEI 6-326908, and HEI 3-089256.
However, the above-described motor control methods have the following problems (1) to (3):
(1) Noise Radiation from a Power Supply
By turning on and off the switches 61 and 62, as set forth above, electric energy to be supplied to loads (motors 51 and 52) is temporally controlled, whereby the speed and torque of the motors 51 and 52 are controlled. However, since the switches 61 and 62 are repeatedly turned on and off, a fluctuation in load viewed from the power supply 71 is temporally great and therefore switching noise (electromagnetic compatibility noise) can readily occur.
(2) Life of Aluminum Electrolytic Capacitors for Smoothing a Power Supply
To prevent switching noise such as that mentioned above, the power supply module 70 contains the smoothing circuit 72 consisting of inductive/capacitive elements, as shown in FIG. 6. Note that not only the smoothing circuit 72 within the power supply module 70 but bypass capacitors on a printed board are considered part of the smoothing circuit 72. As shown in FIG. 8, if the high states of the PWM signals S1 and S2 overlap temporally, the two system motors 51 and 52 (loads) become the load on the power supply 71 instantaneously.
To suppress this instantaneous fluctuation in current, high-frequency components are suppressed by forming the smoothing circuit 72 (which consists of a capacitor, etc.) on a power supply path. As a bypass capacitor for a power supply system, it is common practice to employ an aluminum electrolytic capacitor from the standpoint of capacity and cost. The load to suppress instantaneous current fluctuation will be borne by an aluminum electrolytic capacitor itself. Since an aluminum electrolytic capacitor is a capacitor consisting of two aluminum electrodes separated by an electrolyte, the electrolyte will evaporate gradually if it is used at high temperature for a long period of time. The life depends on the operating environment, but is typically 5 to 10 years. Part of instantaneous current fluctuation energy is converted to heat by the equivalent series resistance (ESR) component of a capacitor. FIG. 9 shows a circuit equivalent to a capacitor. The converted heat accelerates the evaporation of the electrolyte of an aluminum electrolytic capacitor and raises the failure rate of a printed board. In FIG. 9, “ESL” represents an equivalent series inductance and “CAPACITOR” represents a capacitor main body.
(3) Increase in the Required Maximum Current-Carrying Capacity of the Power Supply Due to Simultaneous Operations of Two System Motors
The motors 51 and 52 consume much current during acceleration or deceleration, while during constant-speed operation, they consume only a small amount of current to compensate for friction losses. In order for the two system motors 51 and 52 to perform acceleration or deceleration, the power supply 71 must have enough current-carrying capacity to stand simultaneous acceleration or deceleration. Such an increase in the current-carrying capacity of the power supply 71 adds to costs.
In addition to the above-described problems (1) to (3), there are the following demands:
One important factor that determines the performance of a library apparatus is the speed at which the cartridge (storage medium) 200 is conveyed. To enhance the conveyance speed, it is necessary to simultaneously operate a plurality of motors (motors 51 and 52 in this example) that are equal to or greater than the number of dimensions in the conveying direction. Of course, if the motors 51 and 52 operate simultaneously at the maximum power that the specification of a library apparatus can allow, they can convey the cartridge 200 fastest.
However, when the moving mechanisms in the X-axis and Y-axis directions (horizontal moving mechanism 120 and vertical moving mechanism 130) operate at peak power to convey the cartridge 200 to a target coordinate point (Xp, Yp), that is, as described above, when the two system motors 51 and 52 are digitally controlled by PWM so that they operate simultaneously at peak power, there is little possibility that because there is a difference between the loads of the two moving mechanisms, the cartridge 200 will reach the target X-axis position Xp and target Y-axis position Yp at the same time. That is, there is little possibility that the operations of the two system motors 51 and 52 will finish at the same time.
Therefore, one of the two motors will finish operation earlier and wait for the other motor to finish operation. Since this state is not efficient, it is desirable to increase the consumption efficiency of the power supply and reduce loads, without sacrificing conveyance time (performance), by controlling operations of two system motors so that they end at the same time. Such a technique is not disclosed in any of the aforementioned four patent documents.